In an optical amplifier, a fiber laser device, and the like, a clad pump structure using an amplifying optical fiber having a core doped with rare earth elements is typically employed. The clad pump structure is a structure to which pumping light necessary for amplifying light propagating through a core in an optical fiber propagates and is supplied through a clad.
In order to supply signal light and pumping light to the amplifying optical fiber, an optical fiber coupler is used in some cases. In the optical fiber coupler, a single mode fiber through which the signal light can propagate as single mode light and a multimode fiber through which the pumping light can propagate are fused and drawn to form a fusion-drawn portion.
Patent Document 1 listed below describes an example of an optical fiber coupler and the optical fiber amplifier using the optical fiber coupler. In the optical fiber coupler used for the optical fiber amplifier, a single mode fiber and a multimode fiber are twisted together and fused and drawn to form a fusion-drawn portion. If signal light propagating as single mode light through the single mode fiber and pumping light propagating as multimode light through the multimode fiber are input to the optical fiber coupler, they propagate as the single mode light and the multimode light, respectively, and are input to an amplifying optical fiber from the optical fiber coupler.
Patent Document 1: Japanese Patent No. 2980248